This one is great all year around. You put yoghurt, a ripe banana, and a cup of blueberries into your blender, and just blend for about a minute and a half. If you let it sit for a few minutes, it becomes thicker and thus fills you up nicely.
In summer, you can use frozen blueberries to make the smoothie cooler, or you can add an ice cube or two.
Perhaps you can put a bit of the smoothie into plastic ice-cream cone moulds and freeze them. It makes one healthy and tasty frozen yoghurt / ice cream the next day. Enjoy!